A low shear transfer joint in aero space structure is recognized and defined as a fatigue joint where less than twenty percent (20%) of the fastener shear strength is transferred during design limit loading. This type of joint is common to spanwise aircraft wing attachments, and longitudinal fusilage fasteners.
Evaluation of investigative data brings the conclusion fretting is the common failure mode between one hundred thousand and one million cycles for interference fit low shear transfer joints.
When the load transfer is very low the fretting failure mode is caused by fretting under the collar, nut, or fastener head. For the higher load transfer joints fastener shank fretting is sometimes a failure cause, but faying surface fretting is the primary failure mode.
Fretting is caused by a combination of the following factors:
1. Coefficient of friction between the mated components. PA1 2. Bearing pressure between the mated components. PA1 3. Relative motion between the mated components. PA1 FM=(f.sub.JM /E) d.sub.MAX PA1 E=Modulus of elasticity of joined material PA1 f.sub.JM =Axial stress of joined material PA1 d.sub.MAX =Maximum bearing diameter of the fastener PA1 FM=Fretting motion or elongation PA1 f.sub.BR =Bearing stress PA1 P.sub.T =Fastener preload PA1 A=Maximum bearing area
At the present time applicant does not know the exact interaction equation of these factors, but, by eliminating or changing one or more of these factors fretting failure can be controlled or eliminated. The factor chosen by applicant to control is the relative motion between the fastener and the joint material. In low shear transfer joints, as tension loads are applied to the joined material, specimen stretching occurs, while the fastener remains stationary. This causes relative fretting motion, and the fastener preloads adds to the fretting action.
The equation for relative motion between the fastener head, nut or collar and the joined material is:
When the strain is between 0.002/0.004 in./in. and when the fastener bearing diameter is greater than 1.25 times the fastener shank diameter, fretting failure modes can take place. Since the strain cannot be controlled in the actual structure practically, the bearing diameter has been chosen for control. Fastener head, flush recess, nut and collar installed geometries therefore have been modified to produce bearing diameters of no greater than 1.25 times the fastener shank diameter. The above geometries are devised for fastener preloads which are in the typical range for shear pin and collar fasteners equal to approximately sixty percent (60%) of minimum tensile strength.
The equation for maximum bearing contact equal to 1.25 times the fastener diameter is: ##EQU1## A=Maximum bearing area D=Fastener shank diameter
The bearing stress that correlates to 1.25 maximum bearing diameter is: EQU f.sub.BR =0.60 P.sub.T /A
The 0.60 factor is the average pin and collar preload ratio to fastener minimum ultimate strength.
The bearing stresses using the above equations are well below the yield strengths of common collar/nut or sheet materials, so they indicate stable joints. In order to obtain the minimum bearing diameter contact desired from the nut/collar or fastener head, applicant has chosen a material yielding approach. This approach consists of creating an initial bearing area that is below the bearing area required to sustain yield bearing stresses, and then allowing the collar/nut or sheet material to yield until a stable bearing area is created.
This insures that the minimum bearing diameter is obtained for a given fastener preload condition.
The present application is related to applicant's application Ser. No. 501,872, filed Aug. 30, 1974 and now abandoned.
The state of the prior art is indicated by U.S. Pat. No. 3,094,017 of Champoux et al, U.S. Pat. No. 3,421,562 of Orloff et al and U.S. Pat. No. 2,531,049 of Orloff, the disclosures of which are incorporated herein by reference.